Why do earthquakes happen?

Just like a giant, spherical puzzle, the outer layer of the Earth’s crust is made up of many pieces, known as tectonic plates (see picture below). Earthquakes happen when energy is released as these plates slip past one another.

Despite appearances, the ground beneath our feet is constantly on the move. Underneath it lies the mantle, which consists of warm molten rock. Movement within the mantle is governed by convection, making it comparable to the workings of an enormous lava lamp. Hotter rock near the planet’s core is less dense and moves upwards towards the surface. As the rock cools, it becomes denser and sinks back down.

These currents act like giant conveyor belts, propelling tectonic plates along slowly but surely by up to 10 cm per year, about as fast as your fingernails grow.

 

Where tectonic plates grind past each other, friction sometimes causes two plates to get stuck as their jaggedy edges catch. When this happens pressure builds up, until eventually  the rock gives way and the plates suddenly slip alongside each other with a jolt. The stored energy is released abruptly and vibrations travel in shockwaves through the rock.

These seismic waves radiate in all directions from the point underground where the energy was released, known as the focus. Directly above this is earthquake's epicentre: the point on the earth’s surface where the earthquake will be experienced most strongly.

The result can be anything from an imperceptible tremor to a full blown earthquake, depending on the amount of energy released.

Almost all earthquakes are confined to tectonic plate boundaries, but predicting individual quakes remains almost impossible.  Luckily, of the hundreds of thousands of earthquakes which happen every year, most are so weak that we don’t even notice them.

Find related websites on plate tectonics with physics.org

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